Isolation and characterization of serum-resistant strains ofPseudomonas aeruginosa derived from serum-sensitive parental strains

Anti-Pseudomonas aeruginosa Vaccines and Therapies: An Assessment of Clinical Trials

Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen that causes high morbidity and mortality in cystic fibrosis (CF) and immunocompromised patients, including patients with ventilator-associated pneumonia (VAP), severely burned patients, and patients with surgical wounds. Due to the intrinsic and extrinsic antibiotic resistance mechanisms, the ability to produce several cell-associated and extracellular virulence factors, and the capacity to adapt to several environmental conditions, eradicating P. aeruginosa within infected patients is difficult. Pseudomonas aeruginosa is one of the six multi-drug-resistant pathogens (ESKAPE) considered by the World Health Organization (WHO) as an entire group for which the development of novel antibiotics is urgently needed. In the United States (US) and within the last several years, P. aeruginosa caused 27% of deaths and approximately USD 767 million annually in health-care costs. Several P. aeruginosa therapies, including new antimicrobial agents, derivatives of existing antibiotics, novel antimicrobial agents such as bacteriophages and their chelators, potential vaccines targeting specific virulence factors, and immunotherapies have been developed. Within the last 2-3 decades, the efficacy of these different treatments was tested in clinical and preclinical trials. Despite these trials, no P. aeruginosa treatment is currently approved or available. In this review, we examined several of these clinicals, specifically those designed to combat P. aeruginosa infections in CF patients, patients with P. aeruginosa VAP, and P. aeruginosa-infected burn patients.

Immunoglobulin isotype isolated from human placental extract does not interfere in complement-mediated bacterial opsonization within the wound milieu

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Human placental extract has many applications as a wound healer.

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Immunoglobulin G is a key glycoprotein present in human placental extract.

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Placental IgG (25.2 ± 3.97 μg/ml) did not exert anti-complementary effects.

The wound healing potency of an aqueous extract of placenta can be evaluated through the presence of numerous regulatory components. The presence of glycans was detected by thin layer chromatography and fluorophore-assisted carbohydrate electrophoresis. Mass spectrometric analysis revealed the existence of multiple fragments of immunoglobulin G (IgG). IgG was present in the extract at a concentration of 25.2 ± 3.97 μg/ml. IgG possesses anti-complementary activity by diverting the complement activation from target surface. Thus, effect of placental IgG on complement–bacteria interaction was investigated through classical and alternative pathway and the preparation was ascertained to be safe with respect to their interference in the process of bacterial opsonization.

Binding of protegrin-1 to Pseudomonas aeruginosa and Burkholderia cepacia

BACKGROUND

Pseudomonas aeruginosa and Burkholderia cepacia infections of cystic fibrosis patients' lungs are often resistant to conventional antibiotic therapy. Protegrins are antimicrobial peptides with potent activity against many bacteria, including P. aeruginosa. The present study evaluates the correlation between protegrin-1 (PG-1) sensitivity/resistance and protegrin binding in P. aeruginosa and B. cepacia.

METHODS

The PG-1 sensitivity/resistance and PG-1 binding properties of P. aeruginosa and B. cepacia were assessed using radial diffusion assays, radioiodinated PG-1, and surface plasmon resonance (BiaCore).

RESULTS

The six P. aeruginosa strains examined were very sensitive to PG-1, exhibiting minimal active concentrations from 0.0625-0.5 microg/ml in radial diffusion assays. In contrast, all five B. cepacia strains examined were greater than 10-fold to 100-fold more resistant, with minimal active concentrations ranging from 6-10 microg/ml. When incubated with a radioiodinated variant of PG-1, a sensitive P. aeruginosa strain bound considerably more protegrin molecules per cell than a resistant B. cepacia strain. Binding/diffusion and surface plasmon resonance assays revealed that isolated lipopolysaccharide (LPS) and lipid A from the sensitive P. aeruginosa strains bound PG-1 more effectively than LPS and lipid A from resistant B. cepacia strains.

CONCLUSION

These findings support the hypothesis that the relative resistance of B. cepacia to protegrin is due to a reduced number of PG-1 binding sites on the lipid A moiety of its LPS.

Complement activation and C3 binding by serum-sensitive and serum-resistant strains of Pseudomonas aeruginosa

The relationship among complement consumption, C3 deposition, and C3 fragmentation pattern was compared for serum-sensitive (Sers) and serum-resistant (Serr) strains of Pseudomonas aeruginosa. The Sers strains, which were mucoid strains derived from patients with cystic fibrosis, had lipopolysaccharide deficient in O-antigen side chains. These organisms generally activated much less complement per organism than their Serr counterparts, characterized by the presence of lipopolysaccharide with long lipopolysaccharide O side chains. Surprisingly, however, although the Serr strains consumed more total hemolytic complement, less C3 was deposited onto the surface of these strains than onto that of the Sers strains. Maximal C3 binding required the participation of both the classical and alternative complement pathways, although classical complement pathway involvement was more important for Serr strains. Finally, while more than half of the C3 deposited on most Sers strains was in the form of C3b, most of the C3 on the Serr strains was in the form of iC3b, indicating a more rapid and extensive conversion of C3b to iC3b on the surface of these strains. Limited complement activation by Sers mucoid strains of P. aeruginosa may confer a selective survival advantage to these organisms in colonizing the airways of patients with cystic fibrosis.

Interaction of complement with serum-sensitive and serum-resistant strains of Pseudomonas aeruginosa

The interaction of complement with the following two strains of Pseudomonas aeruginosa was examined: 144M, a mucoid, serum-sensitive strain bearing short lipopolysaccharide O chains, and 144M-SR, a mucoid, serum-resistant strain bearing long lipopolysaccharide O chains isolated by repeated passage of 144M in increasing concentrations of pooled normal human serum (PNHS). While significant killing of 144M occurred in 5 to 40% PNHS, no killing of 144M-SR was observed. Both strains activated complement, especially 144M-SR which consumed 88.7, 96.4, and 100% of the available complement 3 (C3), C5, and C9, respectively, in 10% PNHS during a 60-min incubation at 37 degrees C. Although it activated more C3 than did 144M (54.9% consumption), 144M-SR bound only half as much C3 as 144M. Similarly, although 144M-SR activated more C9 than did 144M (50.0% consumption in 60 min), there was considerably less C9 attached to 144M-SR (2,990 molecules of C9 per bacterium) than to 144M (13,700 molecules per bacterium) after 60 min of incubation. Furthermore, only 162 molecules of the C9 bound to 144M-SR remained bound after treatment with 0.1% trypsin, while 5,692 molecules of the C9 bound to 144M remained bound under similar conditions. These results show that the serum resistance of 144M-SR does not represent a failure to activate complement efficiently, but instead reflects failure of the assembled terminal complement complex C5b-9 to insert stably into the outer membrane of this strain.

[Infections of the respiratory tract with Pseudomonas aeruginosa in cystic fibrosis]

The main cause of death in cystic fibrosis (CF) patients is progressive pulmonary insufficiency frequently associated with chronic infections of the respiratory tract by Pseudomonas aeruginosa. Bacteria of this species synthesize numerous extracellular products contributing to its pathogenicity. An alginate-like exopolysaccharide is characteristic for mucoid mutants predominating among P. aeruginosa isolates from CF patients. It interferes with immune defense mechanisms of the host and probably protects the bacteria against certain antibiotics. Furthermore, it is involved in the formation of bacterial microcolonies that resist mucociliary clearance, opsonisation, and phagocytosis. Exotoxin A and elastase are regarded as the most important among various extracellular enzymes involved in pulmonary injury in CF patients. Exotoxin A inhibits eukaryotic protein synthesis leading to necrosis; elastase, together with other Pseudomonas-proteases, induces hemorrhagic lesions and necrosis and seems to inactivate immunoglobulins and complement factors. Phospholipase C and glycolipid represent two hemolysins of P. aeruginosa that may contribute to cytopathogenic effects in infected lungs. No primary defect in the immunological defense mechanisms of CF patients has been described so far. Antibodies against various P. aeruginosa antigens including those mentioned above have been demonstrated, but a complete elimination of the bacteria from infected lungs has not been observed. Therapy of pulmonary P. aeruginosa infections in CF patients usually includes combinations of antibiotics of the beta-lactam and aminoglycoside type. Difficulties arise from an unusually high intrinsic resistance of P. aeruginosa as well as from poor penetration of many antibiotics into the sputum of CF patients. Therefore, future efforts to manage the Pseudomonas problem in CF will probably concentrate on prophylactic therapy, e.g. childhood vaccination of CF patients in order to prevent bacterial colonization of the respiratory tract.

Serum sensitivity of a Pseudomonas aeruginosa mucoid strain

The susceptibility of Pseudomonas aeruginosa 144M (a mucoid strain isolated from the sputum of a cystic fibrosis patient) to the bactericidal activity of pooled fresh normal human serum (FHS) was examined. FHS at concentrations of greater than or equal to 2.5% was capable of killing greater than 95% of strain 144M. Strain 144M was killed by FHS in a dose-dependent manner. Although either immunoglobulin M (IgM) or IgG was bactericidal in the presence of complement, IgM was about 10 times as effective as IgG. However, optimal killing activity required both IgM and IgG and complement, activated by the classical pathway. A role for lysozyme in the killing of 144M was demonstrated only when low concentrations of FHS were used. In contrast to 144M, P. aeruginosa strains 144NM and 144M(SR) were totally resistant to FHS at all of the concentrations tested (up to 50%). Neither the FHS susceptibility of 144M nor the FHS resistance of 144NM or 144M(SR) was altered by choice of growth medium, growth phase, or temperature of growth. Results of absorption studies with whole organisms, isolated outer membrane preparations, or lipopolysaccharide (LPS) from each strain suggest that the antigen(s) which binds the bactericidal immunoglobulins is accessible on the surface of 144M but not on the surface of 144NM or 144M(SR), is insensitive to trypsin treatment, and is believed to be LPS. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the three LPS preparations demonstrated that 144M LPS contained primarily lipid-A-core polysaccharide components, whereas the LPS from 144NM and 144M(SR) were heterogeneous, with various degrees of O-side-chain substitution. These results suggest that at least one target for bactericidal antibody on the surface of 144M is contained in the rough LPS of this strain.